1. Field of the Invention
The present invention relates to a control structure for the active damping of low-frequency oscillations in numerically-controlled machine tools, having an rpm regulator with a proportional component and an integral component.
2. Discussion of Related Art
Such oscillations considerably affect the surface quality of a treated work piece.
The numerical control of a machine tool controls the processing of a work piece by a parts program, in which an exact treatment process is fixed in the most different treatment cycles. In the course of this, a tool is required to follow a predetermined track as exactly as possible, so that the shape of the finished work piece corresponds to the preset conditions. To this end it is necessary to appropriately control the various shafts of the machine tool with their respective rotary or linear drive mechanisms. In order to be able to maintain a predetermined treatment track, control structures are employed which, in a position regulating device, calculate a nominal speed (for linear drive mechanisms) or nominal rpm (for rotary drive mechanisms) from the respective predetermined nominal position and the actual position of the tool, by which it is intended to correct a possible position deviation. The difference between the nominal rpm and the actual rpm is converted in an rpm regulator into a nominal current for the drive mechanism which, via the motor constant of the drive mechanism, also corresponds to a nominal torque. After a comparison with the actual current, a nominal voltage is determined from this nominal current by regulation in a current regulator and is converted in the drive mechanism amplifier and applied to the phases of the motor. Suitable measuring systems check the actual position of the work piece, from which the actual rpm can be derived. Current sensors in the supply lines to the motor detect the actual current.
The connection between the drive mechanism and the tool is never completely rigid, instead it contains elastic components, which are therefore capable of oscillation. Thus, mechanical resonance frequencies occur, which can lead to undesired oscillations in case of an adverse parameterization of the control structure and/or reduced internal damping of the elastic components. Because of the demand for increasingly greater bandwidths of the control structures, primarily realized by high amplifier factors in the position control circuit, such low-frequency resonance frequencies are also amplified and are superimposed on the tool track. Low-frequency oscillations in the range of up to 50 Hz are clearly visible in the form of an undesired surface waviness of the treated work piece.
A negative phase angle rotation has particularly negative effects in the formation of such resonance oscillations, such as is created in particular by the delays in the control system during the cooperation with the integral component of the rpm regulator. The integral portion can be reduced by reducing the corresponding amplification factor, and the resonance oscillation weakened in the process, but at the same time the rigidity of the machine tool and the quality of the interference removal are also reduced.
Therefore WO 01/23967 A1 describes the parameterization of a regulator system, in which the feedback of the actual rpm to the nominal rpm upstream of the rpm regulator is split onto two summing points, and wherein a reference model in the form of a proportional component with second order delay (PT2 member) is switched into the branch upstream of the integrating element of the rpm regulator. This reference model is matched to the behavior of the closed control circuit without an integral component in the rpm regulator, so that the disadvantageous influence of the integral component does not appear in the guidance behavior of the rpm regulator. This disadvantageous influence is particularly strong because of the negative phase shift by 90 degrees occurring in the integral component. However, only resonance frequencies above approximately 20 Hz can be damped with this arrangement, and in connection with large machines with several resonances this circuit can only positively act on the respectively highest resonance frequency, while lower resonance frequencies possibly are even negatively affected.